The present invention relates to methods and apparatus for measuring the curvature of reflective surfaces such as the surfaces of semiconductor wafers.
Semiconductors are commonly formed by epitaxial growth. In an epitaxial growth process, materials which form a layer, such as a semiconductor layer, are deposited onto the surface of a substrate, typically a crystalline substrate, so that the deposited material forms a generally crystalline structure having a crystal lattice similar to that of the substrate. The spacing between atoms within a crystal lattice (referred to as the “lattice spacing”) depends upon the composition of the crystal. Where the deposited layer has a composition different from the composition of the substrate, the deposited layer may have a nominal lattice spacing, different from the lattice spacing of the substrate. In this case, the deposited crystalline layer forms with its lattice spacing stretched or compressed to conform to the lattice spacing of the substrate. Stated another way, the deposited layer is formed in a strained condition.
This causes the wafer to deform into a dome-like shape. Thus, the surface of the wafer which is originally flat takes the form of a segment of a sphere. For a wafer and layer having a given set of physical properties, there is a known relationship between the degree of curvature and the strain in the deposited layer. It has been proposed heretofore to measure the curvature of a wafer during deposition of a layer thereon so that the strain in the deposited layer can be monitored during the deposition process. See, e.g., Chason et al., “Measurements Of Stress Evolution During Thin Film Deposition,” Materials Research Society Symposium Proceedings Vol. 428 pp. 499-504 (1996). However, the systems proposed heretofore have not been well suited to monitoring strain during wafer growth under typical production conditions. For example, many compound semiconductors, such as III-V semiconductors, are grown using metal organic chemical vapor deposition (“MOCVD”). In some MOCVD processes, numerous wafers are disposed on a carrier which is movably mounted within a deposition chamber. While the carrier and chamber are maintained at an elevated temperature, gases suitable for forming the desired layer are admitted to the chamber. The carrier is moved rapidly within the chamber to promote even distribution of the gases on the surfaces of the numerous wafers and uniform reaction conditions. Merely by way of example, an MOCVD reactor of the type sold under the trademark TurboDisk® by the TurboDisk Division of Veeco Instruments Inc. incorporates a disk-like carrier which rotates at speeds on the order of 1,000 revolutions per minute, so that the wafers move with considerable speed around the axis of rotation of the carrier.
In the apparatus taught in the aforementioned Chason et al. article, a laser beam is subdivided into several parallel beams which are directed onto a surface and reflected back from the surface to a detector. The relative spacing between the beams can be monitored to provide data from which the wafer curvature can be calculated. This apparatus is relatively delicate and expensive, and is typically incapable of measuring the curvature of a fast-moving wafer. Devices of this type also require a large diameter view port, which further limits their use with industrial MOCVD reactors. Cheng, U.S. Pat. No. 5,233,201 and Cheng, U.S. Pat. No. 5,118,955 (“Cheng '201 ” and Cheng '955 ” respectively) disclose variants of a relatively simple, single-beam system. A single beam is swept across the surface of a wafer and reflected back to a detector. However, instruments of this type require precise placement and movement of the wafer, and are not well suited for measure wafer curvature in a production environment.
Thus, despite considerable efforts in the art heretofore, there are substantial needs for improved methods and apparatus for monitoring wafer curvature.